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From a single phase power supply, the low frequency welding control
utilizes an inversed parallel pair of Silicone Controlled Rectifiers
(SCR) to control the output the welding power transformer. SCRs
are turned on by pulsing a gate with a voltage signal. The welding
control uses predictive algorithms to determine the best point to
pulse the gate of the SCRs during the half cycle of the alternating
current supply. The SCRs are turned off only when the alternating
current supply is below the threshold point called "minimum holding
current". This occurs near the zero crossing point of the AC power
supply.
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